1. Field of the Invention
The present invention relates to a method of and an apparatus for inspecting an electrically-conductive pattern on a printed board, and more particularly to an improvement in inspecting a printed circuit board for positional discrepancy between a wiring pattern and a through hole which may be a mini-via hole.
2. Description of Background Arts
As small electronic elements having high performance have been brought to the field of electronic engineering, printed circuit boards on which fine electrically-conductive patterns are formed at a high density have been utilized for mounting and interconnecting these electronic elements. The fine conductive patterns can be obtained by making the width of wiring patterns narrow and by decreasing the diameter of through holes. In particular, through holes having a diameter of 0.1 mm to 0.5 mm are now utilized in multi-layer boards in place of conventional through holes having diameter of 0.8 mm, and the former are called as mini-via holes.
As the diameter of through holes becomes small, required are new techniques in various aspects such as plating treatment, drilling and reliability test of the through holes.
In general, the accuracy in drilling is inferior to that in photoetching process, and therefore, the through holes are often formed at positions deviated form target positions on conductive patterns. However, slight deviation of the through holes from the target positions causes substantially no problem in the electronic reliability of the printed circuit board in the case where the diameter of the through holes is about 0.8 mm, because relatively large lands are provided around the target positions for accepting the through holes.
On the other hands, the diameter of the lands are made small in proportion to the reduction of the diameter of the through holes, and it becomes difficult to always form the through holes in the lands by drilling. Accordingly. positional deviation of the through holes from the lands often affects the electronic reliability of the printed circuit board, and inspection of the circuit board for the positional deviations of the through holes becomes important.
The inspection of printed boards for positional deviation of through holes requires two approaches, one of which is electronic inspection and the other is visual inspection. A conventional apparatus for the visual inspection is operable to detect leak lights from cracks of a metal plate. However, the detection with the conventional apparatus is not satisfactory when the same is applied to the inspection of multi-layer printed board in which relatively large number of layers are laminated. Furthermore, the conventional apparatus is not effective in detecting pattern breaks which are caused by positional discrepancy between through holes and conductive patterns.
In order to show an example of the pattern breaks, FIG. 1 and FIG. 2 are now referred where the positional relations between a land R and a through hole H are illustrated. The center point O of the through hole H in FIG. 1 is located just on the center point of the land R, and therefore, the pattern in FIG. 1 is non-defective. On the other hand, the center point O of the through hole H in FIG. 2 is deviated from that of the land R, and part of the through hole H projects into the exterior of the land R. The size of the projecting part can be expressed by the aperture angle .theta.. That is, it is concluded that the pattern is defective when the aperture angle is larger than a predetermined threshold angle.
FIG. 3 and FIG. 4 illustrate the positional relations between a linear part PL of a wiring pattern and a through hole H. The center point O of the through hole H in FIG. 3 is located at the center line CL of the linear part PL, and the pattern is non-defective. On the other hand, the center point O in FIG. 4 is deviated from the center line CL, and part of the through hole H projects into the exterior of the linear part PL. The size of the projecting part is also expressed by the aperture angle .theta., and the pattern is defective when the aperture angle .theta. is larger than a threshold angle.
Therefore, the detection of the aperture angle .theta. is effective in determining whether the pattern breaks are allowable ones or not, and the same has been performed in conventional techniques by observing the pattern break with human eyes and a magnifying lens. The visual observation with human eyes requires a long time for completion of the visual observation, and the inspection accuracy depends on the skill of an inspector. Consequently, it has been desired to develop a technique for inspecting printed boards which can be applied to an automatic inspection.
A technique for the automatic inspection cannot be established without establishing a method of reading the respective images of through holes and wiring patterns at a high accuracy. However, since the opening edges of through holes have indentations or inclinations at which the optical reflection property is not constant, it has been difficult to binalize the respective images of through holes and lands.